New ebike, trying to understand the top speed limitation mechanism

[Rafc]

Hello Everyone,

I have recently bought my first ebike. I am overall happy with the performance but a little puzzled how the current is being cut-off when the bike reaches top speed around 38 km/h. This top speed is lower around 35 km/h when the battery is half-empty, and higher >40 km/h if the bike is stationary and I just lift the rear wheel in the air. But in all cases the current drops nearly instantly to very low values once the top speed is reached. Then it kind of operates in a pulsed mode with low amplitude.

I have checked the speed limit in the display is maxed out at 60 km/h. I have tried the trick with lowering the wheel size, I have tried increasing the current cut off from 20A to 22A, but not much has changed.

Can anyone here explain to me what is the mechanism behind the current suddenly dropping from max value to low value once the top speed is reached? Any idea how can I remove this limitation?

I feel that the 48V 17.5Ah battery, 22A controller and 1000W rated motor combination still have more juice to push the top speed further, just don't know how to achieve that.

Thanks in advance!

Display comms protocol: https://electricbike.com/forum/foru...62496-dmhc-tc488-communication-protocol/page2

Photo of the bike and controller:

https://imgur.com/a/PczBEph

Website showing the bike specs: https://richbitebike.com/product/ri...ack-yellow-electric-fat-tire-snow-bike-ebike/

A picture showing how the controller is connected to other elements of the bike here: https://richbitebike.com/wp-content/uploads/2019/07/RT-022-Controller-Connection-Diagram-print.pdf

 

[Rafc]

The top speed will depend on the voltage.
As battery drains, your voltage will sag, it does make sense that you're getting a lower top speed.

Hi Timpo, thank you, I think I get this part.

Where I struggle is why the current is suddenly cut off at certain speeds. My hypothesis is that the controller has a built-in current limiter that checks the internal speed sensor inside the motor. If this is the case I would like to reprogram the limiter but don't know where to start

 

[Rafc]

In addition, the higher current limit (going from 20A to 22A) is meaningless if

1) your battery's burst rate is not very high, this is a typical problem with cheap ebikes, and yes, I can speak from experience.
2) the drain is limited by BMS for safety

Do you know what cells you have?
It says LG, but LG what?

Sadly I don't know what cells are inside. The sticker on the battery housing says - Model: CBT-13S5P, bar code: 20180803CBT0174, another bar code: A0725KB07648-174 (don't know how to check this).

 

[Rafc]

I have a very similar experience.

The bike was Juiced CrossCurrent S (It was CrossCurrent Air with S upgrade, but essentially, CrossCurrent S)

The controller was rated at 20A max, 10A continuous.
The battery was 48V 10.4A, Samsung 26F cells.
(The battery was good up to 20.4A continuous in this particular application, however, I don't know the BMS's set up)

So, if you only look at the spec sheet, this should be a safe setup, 20.4A continuous battery should be able to handle 10A continuous controller just fine.

However, after certain speed, the battery couldn't keep up, I would not say it was cutting off like you described, but it was a sudden drop in electric currency.
People here on EBR said that BMS in the battery pack will go into "Safe Mode" to protect the cells if you push the battery too hard.

So, I got the new battery pack from Eunorau (with same Reention Dorado case), but this time, it was Samsung 35E, 48V 14Ah.
It has 32A continuous (limited to 20A continuous by BMS)
With the new battery, it was 20A continuous, 35A 10 min. max and 45A 5 sec. max.

Anyways, the battery spec is way above controller's spec.

According to Micah Toll from ebikeschool.com YouTube channel, realistically, your battery spec should NOT be anywhere near controller's spec.
You really wanna be on "safe side" and have much higher battery spec.

To make a long story short, all my problem was fixed when I got the higher spec Samsung 35E cell pack.
I know it's a different bike and I am not sure if it applies to you, but just sharing my story.

Hmm, that makes a lot of sense, but there is one element that does not add up in my case.

If the problem was on the battery side as you said, and BMS was trying to protect the cells, then it would only manifest itself at high electrical currents, right?

But when I ride downhill let's say 50 km/h just pulled by gravity, without any work from the motor, and try to use the throttle or pedal assist - the current just doesnt flow at all until my speed drops below 40 km/h.

Same thing if I lift the rear wheel into the air and spin it very fast to let's say 45 km/h and turn the throttle off. When I try to turn the motor on again the current doesnt flow, until the wheel slows down.

So that's why I suspect the speed sensor must be involved. Does this make any sense?

 

[Rafc]

Oh this starts to make sense!

I am not 100% sure, but this linked motor looks pretty much exactly like mine.

So the spec you linked says maximum 325 rpm (I guess max peak) and 290 rpm (I guess max constant speed)...

That gives angular speed w = 5.4166 rot/s (peak) and 4.833 rot/s (constant)

Wheel size is 26 inch so radius R = 0.3302 m

That would give linear max speed of v = 2×pi×w×R = 40.5 km/h (peak) and 36.1 km/h (constant).

This is exactly what my motor is doing - shuts down above 40 and pulses just above 36.

Does this make sense to you too?

 

[Rafc]

Ohh..!

I think you reached the motor's structural limit. You need higher voltage battery.

What's your motor?

If we assume I have Bafang RM G060.750.DC motor, would it be very risky to drive it with higher voltage than specified? Eg from 52V battery?

Then again, we are probably looking at 8-10% improvement in top speed, correct? Perhaps not worth investing in another battery for the sake of this improvement only...

 

[Rafc]

Yes, it does make sense.

The only way to increase top speed is getting a 52V battery.
It will typically add 2-3mph, it won't be a drastic change, but it will give you a small increment in top speed.

Thanks man, also for linking the videos, this explains a lot and will definitely help me decide once I am ready for upgrades!

 

[almikel]

I have a feeling you probably have Bafang RM G060.350.DC or something.

Most cheap ebikes (like mine) have overvolted motor.

The only way for you to find it out is open up the motor, unless there's a model # written on the motor casing?

Check out Bolton (ebike tuner) YouTube video.. they opened up Rad Rover's Bafang motor.
It's powered by 350W motor overvolted to 750W.
However, the "real 750W" motor will be a lot more robust.

Micah Toll from EbikeSchool.com talked about this too.

Micah's video makes a lot of sense, and his points on overvolting a motor is analogous to another hobby of mine - home audio, and specifically driving woofers in speakers/subs - woofers have motors just like e-bikes, they just don't rotate.
Woofer drivers are never specified for voltage, but they usually have a power (wattage) rating (eg 250 Watts) - similar to over volting e-bike motors, the wattage ratings of woofers are just a guideline based on arbitrary "typical" usage.

You could apply a high instantaneous voltage to a woofer driver, way exceeding it's max power rating (let's stay with 250W), but as long as the driver's maximum excursion wasn't exceeded, the driver wouldn't be damaged in any way...a 250W driver would easily manage a short 1000W peak as long as the driver's excursion limit wasn't exceeded.
OTOH you could feed the same driver with a much lower voltage continuous signal of far lower power than 250W and easily melt the driver voicecoil after a surprisingly short period of time.

Bottom line - remain sympathetic to your gear - with e-bikes this is primarily monitoring how hot the motor/battery gets.

cheers,
Mike

 

[Stefan Mikes]

Are you aware @almikel the motor of your e-bike peaks at 520 W?

 

[almikel]

Are you aware @almikel the motor of your e-bike peaks at 520 W?

I didn’t know that - cheers

 

[Lord Zene]

Oh this starts to make sense!

I am not 100% sure, but this linked motor looks pretty much exactly like mine.

So the spec you linked says maximum 325 rpm (I guess max peak) and 290 rpm (I guess max constant speed)...

That gives angular speed w = 5.4166 rot/s (peak) and 4.833 rot/s (constant)

Wheel size is 26 inch so radius R = 0.3302 m

That would give linear max speed of v = 2×pi×w×R = 40.5 km/h (peak) and 36.1 km/h (constant).

This is exactly what my motor is doing - shuts down above 40 and pulses just above 36.

Does this make sense to you too?

I love proper arithmetic it never lies. I wish I retained every formula I ever came across.